1. Field of the Invention
This invention relates to a process for selectively chlorinating aromatic compounds in the side chain and, particularly, to a process for chlorinating the 2-alkyl group of 2-alkyl benzoic acid or ester.
2. Related Information
Halogen-containing aromatic compounds are well-known, versatile intermediates. In particular, 2-alkyl benzoic acid or esters which have been chlorinated at the 2-alkyl group, e.g., methyl 2-(chloromethyl)benzoate (MCMB), otherwise known as alpha-chloro-o-toluic acid methyl ester, is a well-known intermediate for a rice herbicide.
The known processes for preparing esters of, for example, chloromethylbenzoic acid, are comparatively complicated and difficult to practice. British Patent Specification No. 773,131, dated Apr. 24, 1957, discloses the preparation of esters of chloromethylbenzene carboxylic acids by the chlorination with gaseous chlorine at a temperature of between 80.degree. C. and 170.degree. C. of esters of p-toluic acid. Japan Kokai No. 7470.946, dated July 9, 1974, as abstracted in Chem. Abstracts 81.135759x, discloses the preparation of methyl p-(monochloromethyl)benzoate by the chlorination of methyl para-toluate with chlorine under UV radiation at 20.degree. C. to 70.degree. C.
Attempts to photochlorinate 2-alkylbenzoic ester, e.g., methyl 2-methylbenzoate (MMB), resulted in difficulties because of thermal instability at relatively low temperatures, for example, 60.degree. C. for MCMB which degraded to a phthalide degradation product. Consequently, attempts to obtain product relatively free of phthalide impurities were frustrated.